Colored glass-ceramics and method

ABSTRACT

A color package is disclosed that provides a light ivory tint in potassium fluorrichterite glass-ceramic materials containing a cristobalite secondary crystal phase. The package includes Fe 2  O 3 , NiO, Se and, optionally, cobalt oxide within limited ranges.

FIELD OF THE INVENTION

The field is glass-ceramic materials and production of colors therein.

BACKGROUND OF THE INVENTION

A glass-ceramic is a material having at least one crystalline phasethermally developed in a uniform pattern throughout at least a portionof a glass precursor. Glass-ceramics have been known for over 30 yearssince being described in U.S. Pat. No. 2,920,971 (Stookey). They findapplication in diverse areas, an area of particular interest being thefabrication of articles used in the preparation and serving of food.Such articles include cookware, bakeware, tableware and flat cooktops.

In general, production of a glass-ceramic material involves three majorsteps:

1. Melting a mixture of raw materials, usually containing a nucleatingagent, to produce a glass.

2. Forming an article from the glass and cooling the glass below itstransformation range.

3. Crystallizing ("ceramming") the glass article by an appropriatethermal treatment. The thermal treatment usually involves a nucleatingstep at a temperature slightly above the transformation range. This isfollowed by heating to a somewhat higher temperature to cause crystalgrowth on the nuclei.

U.S. Pat. No. 4,608,348 (Beall et al.) describes a family ofglass-ceramics having potassium fluorrichterite as a primary crystalphase with a secondary crystal phase of at least 10% by volumecristobalite. The glass-ceramic materials are described as being highlycrystalline, exhibiting superior toughness against fracture propagationand having a modulus of rupture in excess of 20,000 psi.

The glass-ceramics disclosed in the Beall et al. patent havecompositions consisting essentially of, expressed in weight percent onthe oxide basis:

    ______________________________________                                               SiO.sub.2                                                                           65-69                                                                   Al.sub.2 O.sub.3                                                                    0.75-3.5                                                                MgO   13.5-17.5                                                               CaO     3-4.8                                                                 Li.sub.2 O                                                                          0.5-2.0                                                                 Na.sub.2 O                                                                          1.5-3.3                                                                 K.sub.2 O                                                                           4.2-6.0                                                                 BaO     0-2.5                                                                 P.sub.2 O.sub.5                                                                       0-2.5                                                                 F     3.3-5.5                                                          ______________________________________                                    

Optionally, up to 5 mole percent total of compatible metal oxides, suchas, SrO, TiO₂, B₂ O₃, ZrO₂ and ZnO, are contemplated. As₂ O₃ and Sb₂ O₃are suggested as fining agents, and various known colorants may bepresent in conventional amounts.

The crystal phase in a potassium fluorrichterite glass-ceramic is apolymeric chain silicate in which a double, or higher order, multiplechain forms. A characteristic feature is a high MgO content which leadsto rapid crystallization and a low, residual glass content. However,some residual glass content is necessary to avoid shattering of thecrystallized material during cooling.

In addition to high MgO content, the compositions contain relativelyhigh contents of K₂ O, Li₂ O and F, coupled with relatively low Na₂ Oand CaO contents. These composition considerations contribute to lowthermal distortion in a formed glass article during heat treatment toconvert the glass to a glass-ceramic.

The potassium fluorrichterite glass-ceramics have found commercialapplication in the dinnerware field. U.S. Pat. No. 5,070,043 (Amundsonet al.) describes a color package for a pressed cup having potassiumfluorrichterite as a primary crystal phase and cristobalite as asecondary phase in accordance with the Beall et al, teaching. The cuphas a beige color designed to match a laminated dinnerware line soldunder the CORNERSTONE trademark. The color package consists of:

0.065-0.16% Fe₂ O₃

0.055-0.16% NiO

0.0015-0.0029% Se

0-0.0014% Co₃ CO₄

The color package is further defined with reference to a polygonencompassing the x and y color coordinates. The minimum x value is0.3245 and the minimum y value is 0.3333.

The present invention also provides a color package for a potassiumfluorrichterite glass-ceramic in accordance with the Beall et al. patentteaching. In particular, it provides a line of glass-ceramic dinnerwarehaving an attractive, light ivory color.

SUMMARY OF THE INVENTION

The article of the invention is an ivory-colored, opaque, glass-ceramicarticle having potassium fluorrichterite as a primary crystal phase,having a secondary crystal phase of cristobalite amounting to at least10% by volume of the glass-ceramic, and having a color packageconsisting of:

0.045-0.065% Fe₂ O₃

0.015-0.035% NiO

2-8 ppm Se

0-0.0014% CO₃ O₄.

The method is a method of producing an ivory color in an opaqueglass-ceramic article which comprises mixing a batch for a precursorglass capable of being thermally converted to a glass-ceramic having aprimary crystal phase of potassium fluorrichterite and a secondarycrystal phase of cristobalite amounting to at least 10% by volume of theglass-ceramic, incorporating, in the glass batch, materials that providea color package consisting of:

0.045-0.065% Fe₂ O₃

0.015-0.035% NiO

2-8 ppm Se

0-0.0014% CO₃ O₄

melting the batch, forming an article from the glass and heat treatingthe article to form a glass-ceramic having a primary crystal phase ofpotassium fluorrichterite and a secondary crystal phase of cristobaliteamounting to at least 10% by volume of the glass-ceramic.

PRIOR ART

In addition to the patents noted earlier, attention is also directed topending application Ser. No. 08/258,268 filed Jun. 10, 1994 in the nameof W. D. Amundson, Jr., assigned to the assignee of the presentapplication, and entitled CHAMPAGNE COLORED GLASSES. The Amundson, Jr.application describes use of a color package consisting of 0.05-0.25%Fe₂ O₃,>25-175 ppm NiO and >10-100 ppm Se to produce a champagne colorin a soda lime silicate base glass.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE in the accompanying drawing is a graphicalrepresentation of chromaticity coordinates x and y (Illuminant C). TheFIGURE contains a color coordinate box defining the ivory color of thepresent invention in terms of color coordinate limits.

DESCRIPTION OF THE INVENTION

The invention is based in large measure on discovery of a unique colorpackage. The package produces an ivory color in an opaque glass-ceramicas described in the Beall et al.--348 patent. That is a glass-ceramichaving potassium fluorrichterite as its primary crystal phase and asecondary crystal phase of cristobalite amounting to at least 10% byvolume of the glass-ceramic. The color produced is such a light ivorythat it is sometimes referred to as a creamy white.

The term "color package" indicates a combination of certain glasscolorants, customarily metal oxides and/or metals, in certainproportions. These colorants either produce, or control production of, aparticular color in a glass or glass-ceramic material.

The present color package consists of a small amount of selenium (Se)acting in conjunction with oxides of iron (Fe₂ O₃) and nickel (NiO),and, optionally, cobalt (CO₃ O₄). The package components, expressed inpercent by weight or parts per million (ppm) based on the totalglass-ceramic composition, fall within the following ranges:

    ______________________________________                                               Fe.sub.2 O.sub.3                                                                          0.045-0.065%                                                      NiO         0.015-0.035%                                                      Se          2-8 ppm                                                           Co.sub.3 O.sub.4                                                                          0-0.0014%                                                  ______________________________________                                    

The presence of a minute amount of CO₃ O₄, as a tramp impurity in otherbatch materials, is customarily inevitable. A small amount of this oxideusually has little or no effect in an opaque glass-ceramic. Accordingly,up to about 14 ppm is permitted in the present glass-ceramics.

The present color package is generally effective in glass-ceramicscharacterized by a predominant crystal phase of potassiumfluorrichterite. As described in the Beall et al.--348 patent, theseglass-ceramics fall within certain composition ranges. These ranges arerecited in the Background section of this application. They representthe base glass composition ranges for present purposes.

The components of the color package have little or no effect onglass-ceramic properties, other than color. Nor do they have anyappreciable effect on the production methods for the precursor glasses,or the crystallization of those glasses to form glass-ceramics.Accordingly, the teachings of the Beall et al. patent with respect tothese matters are incorporated by reference in their entirety.

The color package may be introduced by incorporating components in theirentirety in the batch fed to a melting unit. However, the package isreadily adaptable to use in a forehearth colorant additive system. Insuch a system, a base glass composition is melted in a melting furnace,and the color package, or a portion thereof, is added as the moltenglass passes through a forehearth.

The method and equipment used to add a colorant material to a moltenglass in a forehearth are referred to as a forehearth coloring system,or a color cell. Such systems (color cells) have been in use for atleast 30 years. They have been used primarily, however, to impart colorsto soda lime glasses, in particular green or blue colors to soda limeglass bottles. Currently, color cells are employed to introduce twotypes of glass colorants: Unmelted, concentrated colorant in particulateform and melted color frits. The former is favored in the United States,while the latter is more popular in Europe.

Early trials demonstrated that an ivory color could be produced in apotassium fluorrichterite-cristobalite type glass-ceramic. The colorpackage employed consisted of 0.049% Fe₂ O₃, 0.029% NiO and 4 ppm Se.However, the resulting article had a slight greenish tint that wasdeemed less than desirable for the ware being designed.

Accordingly, an experimental trial was scheduled with the purpose ofsecuring a wide range of shades from which a color target could beselected. This trial was scheduled during a transition between precursorglasses. The glass being melted before the transition was a precursorglass for a clear white, that is, opaque, glass-ceramic ceramic product.The transition was to a precursor glass for production of abeige-colored product as described by the Amundson, Jr. et al. patent.

Plates and cups were collected at half-hour intervals during a period oftime early in the transition cycle. The cups were glazed with a non-leadglaze and cerammed using a commercial one-step process. The plates werecerammed and glazed separately so that supporting formers could be usedduring ceramming.

Samples of the plates and cups were taken each half hour. They wereanalyzed by x-ray fluorescence diffraction (XRF) for contents of eachcolorant component. Also, colors were measured in terms of x and ycoordinates using the CIE system based on Illuminant C.

TABLE II sets forth selected data measured on the samples. Since changesin both the colorant amounts and the color produced were small, data onsamples taken each hour are reported. Oxides of iron and nickel aregiven in percent while Se and Co₃ O₄ are given in ppm. All measurementswere obtained by x-ray fluorescence procedures on the samples. Thecorresponding color coordinates, as measured employing a Minolta CR-300colorimeter, are also reported.

                  TABLE II                                                        ______________________________________                                        Time  Fe.sub.2 O.sub.3                                                                       NiO     Se   Co.sub.3 O.sub.4                                                                     x      y                                   ______________________________________                                        2:00  0.049    0.010   1    7      0.3080  0.03170                            3:00  0.049    0.012   0    7      0.3085 0.3176                              4:00  0.051    0.015   2    8      0.3097 0.3186                              5:00  0.054    0.018   3    8      0.3108 0.3198                              6:00  0.057    0.022   4    6      0.3116 0.3205                              7:00  0.061    0.028   4    7      0.3131 0.3227                              8:00  0.063    0.034   6    6      0.3148 0.3238                              ______________________________________                                    

In the accompanying drawing, the single FIGURE employs the CIEchromaticity coordinate system to graphically represent the invention.In the FIGURE, x coordinates are plotted on the horizontal axis and ycoordinates are plotted on the vertical axis. The polygon ABCDEFAencompasses coordinate value combinations that provide generallyacceptable colors in accordance with the invention. The color coordinatevalue ranges for this area are:

x=0.3105 to 0.3150

y=0.3200 to 0.3240

The x and y coordinate values for the points on the polygon ABCDEFA areas follows:

    ______________________________________                                                    x     y                                                           ______________________________________                                        A             0.3105  0.3200                                                  B             0.3125  0.3200                                                  C             0.3150  0.3225                                                  D             0.3150  0.3240                                                  E             0.3135  0.3240                                                  F             0.3105  0.3220                                                  ______________________________________                                    

It will be seen from TABLE II, and even more clearly from the drawing,that the Fe₂ O₃ and NiO oxide colorants, and also the x and y colorcoordinates, increase proportionally with the range of acceptable color.Accordingly, a further defining limit of Fe₂ O₃ :NiO weight ratioswithin the ratios of 2:1 to 3:1 can be assigned. Polygon BFGHB, withinthe larger polygon ABCDEFA, is based on samples taken between 6:00 and7:00 hours. The points G and H have x and y coordinate values of:

    ______________________________________                                                    x     y                                                           ______________________________________                                        G             0.3130  0.3226                                                  H             0.3136  0.3220                                                  ______________________________________                                    

The color package for this area, which provides preferred shades ofivory color, is:

0.055-0.062% Fe₂ O₃

0.020-0.028% NiO

4-6 ppm Se.

The 6:30 sample was selected as a tentative preferred target. Thissample has the following measured colorant amounts and colorcoordinates:

0.059% Fe₂ O₃

0.026% NiO

4 ppm Se

7 ppm CO₃ O₄

x=0.3123

y=0.3218

Y=76

The analyzed base glass composition, exclusive of colorants, in weightpercent was:

    ______________________________________                                        67.7%                SiO.sub.2                                                1.6%                 Al.sub.2 O.sub.3                                         14.0%                MgO                                                      4.7%                 CaO                                                      0.2%                 BaO                                                      1.2%                 P.sub.2 O.sub.5                                          3.2%                 Na.sub.2 O                                               4.8%                 K.sub.2 O                                                3.3%                 F                                                         0.03%               Sb.sub.2 O.sub.3                                          0.05%               ZrO.sub.2                                                ______________________________________                                    

The ZrO₂ content was not intentionally added to the batch. Rather, itwas introduced through glass cullet or other batch materials.

The precursor glass was melted in a tank delivering 20 tons per day andoperating at a peak temperature of 1450° C. The cups and plates werecerammed on a normal production schedule. This involved heating to atemperature on the order of 975° C. over a time span of five hours;holding at temperature for three hours; and cooling over a period of twohours.

I claim:
 1. An ivory-colored, opaque, glass-ceramic article havingpotassium fluorrichterite as a primary crystal phase, having a secondarycrystal phase of cristobalite amounting to at least 10% by volume of theglass-ceramic, and having a color package consisting of:0.045-0.065% Fe₂O₃ 0.015-0.035% NiO 2-8 ppm Se 0-0.0014% CO₃ O₄.
 2. A glass-ceramicarticle in accordance with claim 1 having a color defined by x and ycoordinates within the polygon ABCDEFA in the accompanying drawing.
 3. Aglass-ceramic article in accordance with claim 1 having a composition,calculated in weight %, and in addition to the color package, consistingessentially of components that, as calculated in weight percent on anoxide basis, fall within the following ranges:

    ______________________________________                                        65-69%                SiO.sub.2                                               0.75-3.5%             Al.sub.2 O.sub.3                                        13.5-17.5%            MgO                                                       3-4.8%              CaO                                                       0-2.5%              BaO                                                     1.5-3.3%              Na.sub.2 O                                              4.2-6.0%              K.sub.2 O                                               0.5-2%                Li.sub.2 O                                              3.3-5.5%              F                                                         0-2.5%              P.sub.2 O.sub.5.                                        ______________________________________                                    


4. A glass-ceramic article in accordance with claim 1 wherein the colorpackage consists of:0.055-0.062% Fe₂ O₃ 0.020-0.028% NiO 4-6 ppm Se. 5.A glass-ceramic article in accordance with claim 4 having a colordefined by x and y coordinates within the polygon BFGHB in theaccompanying drawing.
 6. A glass-ceramic article in accordance withclaim 3 wherein the composition has the additional limitation that theweight ratio of Fe₂ O₃ :NiO is at least 2:1, but not over 3:1.
 7. Aglass-ceramic article in accordance with claim 3 having essentially thefollowing analyzed composition, exclusive of impurities, in weight %:

    ______________________________________                                        67.7%                SiO.sub.2                                                1.6%                 Al.sub.2 O.sub.3                                         14.0%                MgO                                                      4.7%                 CaO                                                      0.2%                 BaO                                                      1.2%                 P.sub.2 O.sub.5                                          3.2%                 Na.sub.2 O                                               4.8%                 K.sub.2 O                                                3.3%                 F                                                         0.059%              Fe.sub.2 O.sub.3                                          0.026%              NiO                                                        0.0004%            Se                                                         0.0007%            Co.sub.3 O.sub.4.                                        ______________________________________                                    


8. A method of producing an ivory color in a opaque glass-ceramicarticle which comprises mixing a batch for a precursor glass capable ofbeing thermally converted to a glass-ceramic having a primary crystalphase of potassium fluorrichterite and a secondary crystal phase ofcristobalite amounting to at least 10% by volume of the glass-ceramic,incorporating, in the glass batch, materials that provide a colorpackage consisting of:0.045-0.065% Fe₂ O₃ 0.015-0.035% NiO 2-8 ppm Se0-0.0014% CO₃ O₄ melting the batch, forming an article from the glassand heat treating the article to form a glass-ceramic having a primarycrystal phase of potassium fluorrichterite and a secondary crystal phaseof cristobalite amounting to at least 10% by volume of theglass-ceramic,
 9. A method in accordance with claim 8 which comprisesmelting a batch that provides no more than 0.065 weight % Fe₂ O₃ and nomore than 0.0014% CO₃ O₄ in the glass and adding a forehearth additivethat provides 0.015-0.035% NiO and 2-8 ppm Se in the glass.